A typical gas turbine generally includes a compressor, at least one combustor, and a turbine. The compressor supplies compressed air to the combustor. The combustor combusts the compressed air with fuel to generate a heated gas. The heated gas is expanded through the turbine to generate useful work.
Specifically, the gas turbine may include a stator case that defines an exterior of the machine, and a rotor may extend longitudinally through the stator case on the interior of the machine. Within the turbine, a number of turbine blades may be positioned about a disc associated with the rotor, and energy may be transferred to the turbine blades as the heated gas expands. The resulting rotation of the rotor may be transferred to a generator or other load, such that useful work results. The rotation of the rotor also may be employed in the compressor to create the compressed air. For this purpose, a number of compressor blades may be positioned about the rotor in the compressor.
During operation of the gas turbine, the various components of the turbine expand and contract. For example, thermal expansion may occur due to the relatively high temperature associated with turbine operation, and mechanical expansion may occur due to centripetal forces associated with rotation of the interior components.
One problem with gas turbines is that the various components expand and contract at different and varying rates. The varying rates result from differences among the components in material, geometry, location, and purpose. To accommodate for the discrepancy in expansion and contraction rates, a clearance is designed into the gas turbine between the tips of the blades and shroud. The clearance reduces the risk of turbine damage by permitting the blades to expand without contacting the shroud. However, the clearance substantially reduces the efficiency of the turbine by permitting a portion of the heated gas to escape past the blades without performing useful work, which wastes energy that would otherwise be available for extraction. A similar clearance may be designed into the compressor between the compressor blades and the compressor case, which may permit air to escape past the compressor blades without compressing.
The size of the clearance may vary over stages in an operational cycle of the gas turbine, due to varying thermal and mechanical conditions in the gas turbine during these stages. One example operational cycle of a gas turbine is schematically illustrated in FIG. 1. As shown, the gas turbine is typically initiated from a “cold start” by increasing the rotor speed and subsequently drawing a load, which has the illustrated effect on the clearance between the tips of the turbine blades and the turbine shroud. The gas turbine may then be shutdown for a brief period, such as to correct a known issue. During shutdown, the load may be removed, the rotor speed may be reduced, and the components may begin contracting and cooling. Subsequently, a “hot restart” may occur, wherein the gas turbine is restarted before the components return to cold build conditions.
During these operational stages, the clearance may be at a relative minimum at various “pinch points”. For example, the turbine may experience pinch points at full speed, no load (FSNL) and at full speed, full load (FSFL) before the turbine achieves steady state (SS FSFL). The clearances at each of these pinch points may be different during the cold start cycle and the hot restart cycle, with a minimum clearance occurring during the hot restart cycle at full speed, full load. For this reason, the gas turbine is designed with cold build clearances selected to accommodate the limiting point at hot restart full speed, full load, which results in the turbine running with inefficiently large clearances at steady state. In other words, the cold build clearances are selected in view of preventing tip rub during the hot restart cycle and not in view of achieving maximum efficiency during cold start and steady state operations.
The tight clearances observed during the hot restart cycle may be due in part to the gas turbine cooling relatively faster on the exterior (stator) than the interior (rotor) during shutdown. For example, the interior components of the turbine may remain warm, while the stator case may cool and contract toward the interior. The cooling of the stator case may be exacerbated by a cooling air flow traveling along the length of the gas turbine during shutdown. More specifically, the gas turbine may have a series of inlet guide vanes positioned along the compressor, which permit air to enter the gas turbine for compression and subsequent expansion. Because these inlet guide vanes may remain open during shutdown, air may continue to pass into the compressor. The air may be pulled along the length of the gas turbine with continued rotation of the rotor, which is required due to its mass. The resulting draft may further cool the stator case during shutdown, thereby resulting in tighter clearances on hot restart.
What the art needs are systems and methods for reducing differences in thermal response between stator and rotor components during gas turbine operating cycles, particularly the shutdown cycle. The art further needs such systems and methods, which may be implemented on existing gas turbines without adding a substantial number of parts or substantially redesigning the hot gas path.